Fusarium wilt of lettuce continues to be confirmed in numerous coastal California plantings, and previously un-infested ranches are found to harbor the pathogen. Symptoms are most often seen on diseased iceberg (“head”) lettuce, though romaine and other types can also be infected and show symptoms. In fact, some romaine plantings in Yuma (last winter) and coastal California (2018) experienced significant losses from Fusarium wilt.
The Fusarium wilt pathogen can infect lettuce at different stages of development. Lettuce infected at a young stage will be stunted, of poor quality, and fail to reach maturity. Such plants will develop yellow leaves that eventually can turn brown and dry. Plants infected early may die. Lettuce infected at a later stage can form heads; however, such plants will have yellow and then brown foliage, will fail to reach full size, and will be of poor quality. The most telling symptom is the reddish brown discoloration of internal taproot and crown tissues. If subjected to warmer temperatures and water stress, Fusarium wilt symptoms will show up earlier and more severely.
Like most Fusarium wilt pathogens, the lettuce Fusarium wilt fungus has a narrow host range and only causes disease in lettuce. If a field is infested with the lettuce Fusarium pathogen, growers can still rotate with celery, spinach, pepper, tomato, strawberry, and other crops without concern about this specific pathogen. Likewise, if a field has a history of the respective Fusarium wilts of celery, pepper, strawberry, or other crops, these pathogens will not infect lettuce planted in rotation.
With lettuce, Fusarium wilt can be confused with other diseases. Verticillium wilt of lettuce is another vascular disease that shares some of the symptoms caused by Fusarium. The non-parasitic disorder caused by ammonium toxicity causes symptoms that are virtually identical with those of Fusarium wilt. Even some virus disease symptoms (lettuce dieback caused by Tomato bushy stunt virus strains, impatiens necrotic spot caused by INSV) have been confused with Fusarium wilt symptoms. Accurate diagnosis cannot be achieved without conducting laboratory tests. Contact TriCal Diagnostics for help with disease diagnosis and for additional information on this disease.
Photo 1. Fusarium wilt can be a significant economic factor on lettuce.
Photo 2. The Fusarium wilt pathogen can infect lettuce at various developmental stages.
Photo 3. If infected early, lettuce with Fusarium wilt will not reach full size (healthy plant on left).
Photos 4 and 5. A key symptom of Fusarium wilt is the reddish brown discoloration of the internal tissues of the taproot and crown.